1. Field of the Invention
The present invention relates to a method of processing a sensing element of an oxygen concentration sensor, to effectively improve stability in the sensing accuracy, and the oxygen concentration sensor having the thus processed sensing element.
2. Discussion of the Prior Art
A sensor which uses an oxygen-ion conductive solid electrolyte such as a zirconia ceramic and which operates according to the principle of an oxygen concentration cell is known as an oxygen concentration sensor for determining the oxygen concentration of a measurement gas such as a combustion gas within an industrial boiler or an exhaust gas emitted by an internal combustion engine.
Laid-open Publication No. 59-190652 of unexamined Japanese patent application shows an example of such an oxygen concentration sensor of a so-called double-cell type, which is capable of dealing with both an oxidizing measurement gas and a reducing measurement gas. This double-cell type oxygen sensor has an electrochemical oxygen sensing cell and an electrochemical oxygen pumping cell which are formed as an integral sensing element. The oxygen sensing cell has an internal measurement gas space into which a measurement gas in introduced from an external space under a suitably predetermined diffusion resistance. The sensing cell includes a measuring electrode exposed to the measurement gas in the internal measurement gas space, and a reference electrode exposed to a reference gas. An electromotive force is induced between the measuring and reference electrodes, according to the principle of an oxygen concentration cell. The oxygen pumping cell is operated by a controlled pumping current applied thereto, to effect an oxygen pumping action for controlling the oxygen concentration within the internal measurement gas space to which the measuring electrode is exposed, so that the electromotive force inducted by the oxygen sensing cell coincides with a predetermined value. The oxygen concentration of the measurement gas (state of the oxidizing or reducing atmosphere) is determined based on the level of the pumping current at which the electromotive force coincides with the predetermined value.
Theoretically, the oxygen concentration of the measurement gas is represented by the pumping current applied to the pumping cell in the double-cell type oxygen concentration sensor as described above. In practical use of the sensor, however, the detected pumping current does not accurately reflect the oxygen concentration of the measurement gas. Namely, the sensor suffers from some measuring error, or a fluctuation in the output characteristic, and is not satisfactory in the stability of measuring accuracy.
It is also noted that the theoretical or notional relationship between the oxygen concentration of the measurement gas and the detected pumping current of the pumping cell is greatly influenced by the temperature of the sensing element of the sensor. That is, the output characteristic of the sensor tends to vary with the temperature. Further, the output characteristic easily changes when the measurement gas contains CO, H.sub.2 O or other substances which easily adhere to the sensing element.
On the other hand, there is recently an increasing requirement for improved operating stability of such oxygen concentration sensor, in the field of controlling an internal combustion engine of a motor vehicle, keeping pace with an increasing requirement for improved accuracy of detecting the air/fuel ratio of an air-fuel mixture supplied to the engine. Thus, there exists a need for improvement in the known oxygen concentration sensor as described above.